1. Field of the Invention
This invention relates to an optical wavelength converter element for converting a fundamental wave into a second harmonic wave and, more particularly, an optical wavelength converter element in which periodic domain reversals are defined on a ferroelectric crystal possessing a nonlinear optical effect, and a fabricating method thereof.
2. Description of the Prior Art
A proposal has already been made by Bleombergen et al. in Physics Review vol. 127, No. 6 in 1918 (1962), in which the wavelength of a fundamental wave is converted into a second harmonic wave using an optical wavelength converter element with regions (domains) where the spontaneous polarization of a ferroelectric substance possessing a nonlinear optical effect is periodically inverted.
In this method, the fundamental wave call be phase matched with the second harmonic wave by setting the period of .LAMBDA. the domain reversals to be an integral multiple of a coherence length .LAMBDA.c which is given by EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
where .LAMBDA..beta.(2.omega.) designates the propagation constant of the second harmonic wave, and 2.beta. (.omega.) represents the propagation constant of the fundamental wave. When wavelength conversions are effected using a bulk crystal made of a nonlinear optical material, a wavelength to be phase-matched is limited to a specific wavelength inherent to the crystal. However, in accordance with the above described method, phase matching can be efficiently carried out by selecting a period .LAMBDA. which satisfies the condition (1) for an arbitrary wavelength.
In the foregoing optical wavelength converting element made of the ferroelectric substance, it has been heretofore known that a ferroelectric,crystal suffers from optical damage caused by a resulting wave, whose wavelength is converted, such as a second harmonic wave. For instance, in the case of an optical wavelength converting element in which a crystal of LiNbO.sub.3 (LN) is used as a ferroelectric substance, and periodic domain reversals are formed on that crystal, the crystal may be optically damaged by a second harmonic wave having an output of 2 mW (a wavelength of 477 nm). It cannot be said that the optical wavelength converter element which is susceptible to optical damages by a wavelength-converted wave having such a low output is so practically valuable.
For this reason, in order to improve an optical damage threshold value, it is put forward that metal is deposited on the surface of a ferroelectric substance in such a manner as to extend along the direction of the arrangement of domain reversals so that the deviation of electric charges can be eliminated by this metal having a high electric conductivity.
However, in an optical waveguide type optical wavelength converter element, when the above-mentioned metal is deposited on the surface of a ferroelectric crystal which constitutes an optical waveguide, a leakage part of a fundamental wave which travels and a wavelength-converted wave is affected by reflection and scattering, or the like, caused by this metal, leading to light propagation losses. There arises a problem that these light propagation loses bring about a drop in the efficiency of wavelength conversion. Also, in a bulk crystal type optical wavelength converter element, particularly when the element is made of a thin ferroelectric crystal substrate, the end of a beam such as a fundamental wave and a wavelength-converted wave which passes through the substrate similarly undergoes influences such as reflection and scattering caused by metal on the surface of the crystal, thereby leading to a drop in the efficiency of wavelength conversion.